1. Field of the Invention
This invention relates to a system for sensing the image of an object to be photographed and more particularly to an image sensing system including control means for controlling the exposure time of the system.
2. Description of the Prior Art
There have been known shutters of the kind called rotary shutters. The rotary shutter has two shutter blades which are arranged to turn in the same direction at the same speed and consist of light blocking parts and light transmitting parts. The exposure time of the shutter is controlled by controlling the opening angle of the part where these light transmitting parts overlap each other.
FIG. 1 of the accompanying drawings shows the arrangement of the conventional rotary shutter in an exploded oblique view. The shutter arrangement includes an image sensing part 30 which is a charge coupled device (CCD), a camera tube, 8 mm film or the like; a driving motor 31; a shaft 32 of the motor 31 which is provided with a groove 32a; and shutter blades 33 and 34 which are made of thin plates and are formed into a semicircular shape. The shutter blade 33 is secured to the shaft 32 and turns in the direction of the arrow when a current is supplied to the motor 31. An opening angle adjustment shaft 35 is provided with a piercing hole 35a and is rotatably attached to the shaft 32. One end 35c of the angle adjustment shaft 35 is secured to the shutter blade 34. The shaft 35 is further provided with a cam slot 35b. A slider 36 has a piercing hole 36a and is rotatably mounted on the opening angle adjustment shaft 35 by this hole 36a. The slider 36 also has a pin 37 mounted thereon. The pin 37 extends inwardly to enter the groove 32a via the cam slot 35b. A control plate 38 is provided with holes 38a and 38b and is arranged to be shiftable along a guide shaft 40 by virtue of these holes. To the control plate 38 is secured a pin 39 which engages an outer circumferential groove 36b of the slider 36.
The conventional rotary shutter operates as follows: When a current is supplied, the motor 31 causes the shutter blade 33 which is secured to the shaft 32 to turn in the direction of arrow as shown. The rotation of the motor 31 is transmitted to the opening angle adjustment shaft 35 via the groove 32a, pin 37 and cam slot 35b. This causes the other shutter blade 34 to turn also in the direction of the arrow. If the control plate 38 is moved rightward as viewed on the drawing, the pin 37 is caused to move also rightward via the pin 39 and the groove 36b. As a result, the shutter blade 34 is caused via the cam slot 35b and the groove 32a to turn to the left relative to the shutter blade 33 (in the direction of the arrrow) to make the opening angle smaller and, accordingly, to shorten shutter time. Further, if the control plate 38 is moved leftward as viewed on the drawing, an operation which is the reverse of the above causes the shutter blade 34 to turn rightward relative to the other shutter blade 33 in a direction reverse to the direction of the arrow shown in the drawing. This results in a larger opening angle to make the shutter time longer. The shutter is thus arranged to permit control over the exposure time in this manner.
As will be understood from the above description, the conventional rotary shutter necessitates use of many parts for its structural arrangement and thus also results in a complex structure. The complex structural arrangement then tends to have excessive play or back-lash and wear, etc. which degrade the precision, reliability and the service life of the shutter.